1. Field of the Invention
The present invention relates to a crankshaft for an internal combustion engine. More particularly, the present invention relates to a crankshaft including a crank web configured as a helical drive gear, and also including a two-piece crank web including a removable annular weight.
2. Description of the Background Art
Many different designs for internal combustion engines are known, and many different engines are commercially available. In a motorcycle engine, for example, a piston is reciprocated by heat energy obtained by burning an air-fuel mixture. Reciprocal motion of the piston is converted, via a connecting rod, to rotary motion of the crankshaft.
It has been known to form a crankshaft used in a motorcycle engine with an integral spur gear formed on the crankshaft at one side of a crankpin. This known crankshaft is formed with a balance weight mounted on the crankshaft at the other side of the crankpin (see, for example, the crankshaft disclosed in Japanese published patent document JP-A-8-93746).
In the known crankshaft described in Japanese published patent document JP-A-8-93746, a crank web is divided into two main parts, i.e., an integral balance weight mounting portion and a removable balance weight. The balance weight is made removable in order to avoid interference with a machining tool used for forming the gear on one side of the crankpin during manufacture.
In other words, in the design of the known crankshaft, the balance weight mounting portion is formed in a size which can avoid interference with the machining tool when machining the gear, so that the gear is formed by machining at the first balance weight mounting portion, while avoiding interference with the machining tool. After the gear has been machined, the balance weight is then mounted to the balance weight mounting portion.
In the known crankshaft described above, although one of the crank webs is configured as a spur gear, variations of load exerted on each tooth are significant, and operation of the gear may be noisy.
Moreover, since it is generally necessary to match an inertia moment of the crankshaft to respective engines having different properties, in the known crankshaft including the structure described above, it is difficult to share the crankshaft among a plurality of engine types. However, if it were possible to share a crankshaft among a plurality of engine types, the overall cost could be significantly reduced.
Although the known crankshafts have some utility for their intended purposes, a need still exists in the art for an improved crankshaft for an engine. In particular, there is a need for a crankshaft, including an integrally formed drive gear on a crank web thereof, which exhibits superior performance to that obtained with a spur gear.
Further, there is a need for an improved crankshaft that may be adapted to be used in any one of a plurality of engines, so that cost savings and reduction in component numbers may be attained.